Recent technological advances in remote sensing have enabledinvestigation of the morphodynamics and hydrodynamics of large rivers.However, measuring topography and flow in these very large rivers istime consuming and thus often constrains the spatial resolution andreach-length scales that can be monitored. Similar constraints exist forcomputational fluid dynamics (CFD) studies of large rivers, requiringmaximization of mesh-or grid-cell dimensions and implying a reduction inthe representation of bedform-roughness elements that are of the orderof a model grid cell or less, even if they are represented in availabletopographic data. These ``subgrid'' elements must be parameterized, andthis paper applies and considers the impact of roughness-lengthtreatments that include the effect of bed roughness due to``unmeasured'' topography. CFD predictions were found to be sensitive tothe roughness-length specification. Model optimization was based onacoustic Doppler current profiler measurements and estimates of thewater surface slope for a variety of roughness lengths. This proveddifficult as the metrics used to assess optimal model performancediverged due to the effects of large bedforms that are not wellparameterized in roughness-length treatments. However, the generalspatial flow patterns are effectively predicted by the model. Changes inroughness length were shown to have a major impact upon flow routing atthe channel scale. The results also indicate an absence of secondaryflow circulation cells in the reached studied, and suggest simplertwo-dimensional models may have great utility in the investigation offlow within large rivers. Citation: Sandbach, S. D. et al. (2012),Application of a roughness-length representation to parameterize energyloss in 3-D numerical simulations of large rivers, Water Resour. Res.,48, W12501, doi: 10.1029/2011WR011284.